| Message ID | 1597284810-17454-3-git-send-email-chinwen.chang@mediatek.com (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
| Series | Try to release mmap_lock temporarily in smaps_rollup | expand |
On 13/08/2020 03:13, Chinwen Chang wrote: > smaps_rollup will try to grab mmap_lock and go through the whole vma > list until it finishes the iterating. When encountering large processes, > the mmap_lock will be held for a longer time, which may block other > write requests like mmap and munmap from progressing smoothly. > > There are upcoming mmap_lock optimizations like range-based locks, but > the lock applied to smaps_rollup would be the coarse type, which doesn't > avoid the occurrence of unpleasant contention. > > To solve aforementioned issue, we add a check which detects whether > anyone wants to grab mmap_lock for write attempts. > > Change since v1: > - If current VMA is freed after dropping the lock, it will return > - incomplete result. To fix this issue, refine the code flow as > - suggested by Steve. [1] > > [1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@arm.com/ > > Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com> Reviewed-by: Steven Price <steven.price@arm.com> > --- > fs/proc/task_mmu.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++- > 1 file changed, 55 insertions(+), 1 deletion(-) > > diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c > index dbda449..23b3a447 100644 > --- a/fs/proc/task_mmu.c > +++ b/fs/proc/task_mmu.c > @@ -853,9 +853,63 @@ static int show_smaps_rollup(struct seq_file *m, void *v) > > hold_task_mempolicy(priv); > > - for (vma = priv->mm->mmap; vma; vma = vma->vm_next) { > + for (vma = priv->mm->mmap; vma;) { > smap_gather_stats(vma, &mss); > last_vma_end = vma->vm_end; > + > + /* > + * Release mmap_lock temporarily if someone wants to > + * access it for write request. > + */ > + if (mmap_lock_is_contended(mm)) { > + mmap_read_unlock(mm); > + ret = mmap_read_lock_killable(mm); > + if (ret) { > + release_task_mempolicy(priv); > + goto out_put_mm; > + } > + > + /* > + * After dropping the lock, there are three cases to > + * consider. See the following example for explanation. > + * > + * +------+------+-----------+ > + * | VMA1 | VMA2 | VMA3 | > + * +------+------+-----------+ > + * | | | | > + * 4k 8k 16k 400k > + * > + * Suppose we drop the lock after reading VMA2 due to > + * contention, then we get: > + * > + * last_vma_end = 16k > + * > + * 1) VMA2 is freed, but VMA3 exists: > + * > + * find_vma(mm, 16k - 1) will return VMA3. > + * In this case, just continue from VMA3. > + * > + * 2) VMA2 still exists: > + * > + * find_vma(mm, 16k - 1) will return VMA2. > + * Iterate the loop like the original one. > + * > + * 3) No more VMAs can be found: > + * > + * find_vma(mm, 16k - 1) will return NULL. > + * No more things to do, just break. > + */ > + vma = find_vma(mm, last_vma_end - 1); > + /* Case 3 above */ > + if (!vma) > + break; > + > + /* Case 1 above */ > + if (vma->vm_start >= last_vma_end) > + continue; > + } > + /* Case 2 above */ > + vma = vma->vm_next; > } > > show_vma_header_prefix(m, priv->mm->mmap->vm_start, >
On Wed, Aug 12, 2020 at 7:13 PM Chinwen Chang <chinwen.chang@mediatek.com> wrote: > smaps_rollup will try to grab mmap_lock and go through the whole vma > list until it finishes the iterating. When encountering large processes, > the mmap_lock will be held for a longer time, which may block other > write requests like mmap and munmap from progressing smoothly. > > There are upcoming mmap_lock optimizations like range-based locks, but > the lock applied to smaps_rollup would be the coarse type, which doesn't > avoid the occurrence of unpleasant contention. > > To solve aforementioned issue, we add a check which detects whether > anyone wants to grab mmap_lock for write attempts. I think your retry mechanism still doesn't handle all cases. When you get back the mmap lock, the address where you stopped last time could now be in the middle of a vma. I think the consistent thing to do in that case would be to retry scanning from the address you stopped at, even if it's not on a vma boundary anymore. You may have to change smap_gather_stats to support that, though.
On Fri, 2020-08-14 at 01:35 -0700, Michel Lespinasse wrote: > On Wed, Aug 12, 2020 at 7:13 PM Chinwen Chang > <chinwen.chang@mediatek.com> wrote: > > smaps_rollup will try to grab mmap_lock and go through the whole vma > > list until it finishes the iterating. When encountering large processes, > > the mmap_lock will be held for a longer time, which may block other > > write requests like mmap and munmap from progressing smoothly. > > > > There are upcoming mmap_lock optimizations like range-based locks, but > > the lock applied to smaps_rollup would be the coarse type, which doesn't > > avoid the occurrence of unpleasant contention. > > > > To solve aforementioned issue, we add a check which detects whether > > anyone wants to grab mmap_lock for write attempts. > > I think your retry mechanism still doesn't handle all cases. When you > get back the mmap lock, the address where you stopped last time could > now be in the middle of a vma. I think the consistent thing to do in > that case would be to retry scanning from the address you stopped at, > even if it's not on a vma boundary anymore. You may have to change > smap_gather_stats to support that, though. Hi Michel, I think I got your point. Let me try to prepare new patch series for further reviews. Thank you for your suggestion :) Chinwen
diff --git a/fs/proc/task_mmu.c b/fs/proc/task_mmu.c index dbda449..23b3a447 100644 --- a/fs/proc/task_mmu.c +++ b/fs/proc/task_mmu.c @@ -853,9 +853,63 @@ static int show_smaps_rollup(struct seq_file *m, void *v) hold_task_mempolicy(priv); - for (vma = priv->mm->mmap; vma; vma = vma->vm_next) { + for (vma = priv->mm->mmap; vma;) { smap_gather_stats(vma, &mss); last_vma_end = vma->vm_end; + + /* + * Release mmap_lock temporarily if someone wants to + * access it for write request. + */ + if (mmap_lock_is_contended(mm)) { + mmap_read_unlock(mm); + ret = mmap_read_lock_killable(mm); + if (ret) { + release_task_mempolicy(priv); + goto out_put_mm; + } + + /* + * After dropping the lock, there are three cases to + * consider. See the following example for explanation. + * + * +------+------+-----------+ + * | VMA1 | VMA2 | VMA3 | + * +------+------+-----------+ + * | | | | + * 4k 8k 16k 400k + * + * Suppose we drop the lock after reading VMA2 due to + * contention, then we get: + * + * last_vma_end = 16k + * + * 1) VMA2 is freed, but VMA3 exists: + * + * find_vma(mm, 16k - 1) will return VMA3. + * In this case, just continue from VMA3. + * + * 2) VMA2 still exists: + * + * find_vma(mm, 16k - 1) will return VMA2. + * Iterate the loop like the original one. + * + * 3) No more VMAs can be found: + * + * find_vma(mm, 16k - 1) will return NULL. + * No more things to do, just break. + */ + vma = find_vma(mm, last_vma_end - 1); + /* Case 3 above */ + if (!vma) + break; + + /* Case 1 above */ + if (vma->vm_start >= last_vma_end) + continue; + } + /* Case 2 above */ + vma = vma->vm_next; } show_vma_header_prefix(m, priv->mm->mmap->vm_start,
smaps_rollup will try to grab mmap_lock and go through the whole vma list until it finishes the iterating. When encountering large processes, the mmap_lock will be held for a longer time, which may block other write requests like mmap and munmap from progressing smoothly. There are upcoming mmap_lock optimizations like range-based locks, but the lock applied to smaps_rollup would be the coarse type, which doesn't avoid the occurrence of unpleasant contention. To solve aforementioned issue, we add a check which detects whether anyone wants to grab mmap_lock for write attempts. Change since v1: - If current VMA is freed after dropping the lock, it will return - incomplete result. To fix this issue, refine the code flow as - suggested by Steve. [1] [1] https://lore.kernel.org/lkml/bf40676e-b14b-44cd-75ce-419c70194783@arm.com/ Signed-off-by: Chinwen Chang <chinwen.chang@mediatek.com> --- fs/proc/task_mmu.c | 56 +++++++++++++++++++++++++++++++++++++++++++++++++++++- 1 file changed, 55 insertions(+), 1 deletion(-)